In order to properly prime an inkjet printhead (during initial filling, after de-priming, or due to air ingestion), the incoming ink must displace all the air in the ink cavity, because residual air bubbles create a condition in the system that disrupts jetting. Instead of the generated pressure pulse being used to create drops, a large portion of it is absorbed by the volume change of air bubbles. This places extra constraints on the nozzle placement, because all of the air must escape before the liquid reaches the nozzle and blocks the nozzle. If, for example, the ink cavity is long and narrow, the nozzle would have to be placed near the very distant end, or else an air bubble will be trapped at the dead end. Typically, this location of the nozzle is not the optimal position for the nozzle. Also, if the nozzle is placed in a position where clearing the air out is difficult, large purge masses are required to get out all of the air. Large purge masses waste expensive ink, thereby making the nozzle position undesirable.
It would, therefore, be desirable to facilitate the purging of air from the internal ink path without interfering with optimal nozzle placements.